Regular treatment of the condensate water of power plants is necessary for preventing corrosion and maintaining efficient water flow rates. Condensate water commonly contains dissolved and suspended materials, especially oxidized iron commonly referred to as "crud". If not maintained at low levels, crud can build up within the steam loop of the power plant causing pressure build up, reduced flow rates, reduced plant efficiency and expensive repairs. Proper crud removal is particularly important in boiling water nuclear reactors (BWRs) where crud can become radioactive as a result of passing through the "hot" side of the steam loop.
U.S. Pat. No. 4,975,201 (Re. 34,112), reissued Oct. 27, 1992 (incorporated herein by reference), discloses processes for treating power plant condensate water by contacting the water with a mixed bed ion exchange resin comprising cation and anion microporous copolymer beads. These beads include an interpenetrating network of polymer components made by way of known suspension "seeded" polymerization methodologies. As described, the copolymer beads may be made by a variety of seeded polymerization techniques including insitu-type single and second stage processes. These techniques typically include forming polymeric seed particles (e.g., a first polymer component), suspending the seeds in a suitable suspension medium and continuously adding ("con-add") a polymerizable monomer (e.g., a second polymer component) thereto, thereby forming an interpenetrating polymer network. In one alternative approach, the seeds are imbibed with a monomer mixture (e.g., a third polymer component) which is substantially polymerized prior to the subsequent addition of the second polymer component noted above. As indicated, suspension polymerization methodologies are well known in the art, see for example U.S. Pat. No. 4,564,644, which is incorporated herein by reference. These resins have been found to have a high capacity for removing crud from condensate water in BWR nuclear power plants.
One drawback with current cation exchange resins is that they degrade with time to release a variety of sulfonated organic compounds. The mechanism for this resin degradation is believed to be due to an oxidative attack at the benzylic carbon groups of the resins. Subsequent chemical reactions cause the breakage of the copolymer chains. When two or more breakages occur between crosslinked groups, sulfonated organic compounds, i.e., "leachables", are formed which can diffuse out of the resin and into the surrounding water. The molecular weight of these leachables is typically between 150 and 100,000 daltons. Under the high operating temperatures of the steam loop, these leachables can subsequently desulfonate and release highly corrosive inorganic sulfate into the water.
Anion exchange resins are effective at removing cation exchange resin leachables from solution so long as the total amount of leachables are low and so long as the molecular weight of the leachables is relatively low, i.e., below 10,000 (more preferably below about 5,000, and most preferably below about 1,000). Higher molecular weight leachables from the cation exchange resin are not effectively removed from solution by an anion exchange resin. As such, these leachables remain in the process water as an impurity. Additionally, high molecular weight leachables adsorbed by an anion exchange resin can lead to decreased kinetic performance of the anion exchange resin. Thus, it is desirable to reduce the average molecular weight of the cation exchange resin leachables while minimizing the total amount of leachables released into the process stream.
One way to reduce the total amount of leachables released by the cation exchange resin is to incorporate an antioxidant into the resin as described in U.S. Pat. No. 4,973,607; however, improved cation exchange resins are sought which release lower molecular weight leachables, and which release lower amounts of total leachables even without the addition of such antioxidants. It is further desired to provide cation exchange resins which provide improved crud removal properties.
Another approach to reducing leachables from cation exchange resins is to increase the extent of crosslinking of each polymer component used therein. Although the use of increased crosslinking does not reduce oxidative attack of benzylic carbon atoms, it does reduce the likelihood that any given cleavage of a benzylic bond will lead to a leachable species. Unfortunately, with conventional exchange resins the degree of crosslinking and capacity for crud removal are somewhat inversely related. Thus, as the extent of crosslinking is increased to reduce leachables, capacity for crud removal is compromised.